Pit-lake water quality in coal deposits, Southland, New Zealand : a comparison between the lignite and sub-bituminous coal deposits of Southland

Mulliner, Timothy.

Southland contains over 70 % of New Zealand's recoverable coal resources. This study investigates and compares water-quality in pit-lakes associated with coal mining, in the sub-bituminous and lignite deposits of Southland. Water and waste-rock samples were collected from, and in close vicinity to one pit-lake at the Ohai Coal Mine (sub-bituminous), and four pit-lakes at the Newvale/Goodwin Coal Mine (lignite) from October 2005 to July 2006. Major and trace element analysis of wasterock showed no major differences between the two deposits. Mineralogical analysis showed a higher abundance of clay minerals- kaolinite and montmorillonite, in the sub-bituminous waste-rock than in the lignite waste-rock. No sulphide minerals were present in waste-rock samples collected from either site, but the presence of gypsum, jarosite, and iron oxyhydroxide (HFO) at Newvale suggests that sulphides are present and have led to the localised production of acid mine drainage (AMD). Both the maximum potential acidity (MPA) and acid neutralisation capacity (ANC) of waste-rock from both sites were low. Pit-lake pH was lower in the lignite pit-lakes {ea. 4-7) than in the sub-bituminous pit-lake (ea. 7 -9), and was associated with higher concentrations of dissolved AI, Mn, Ni and Zn (2.65, 2.28, 0.42 and 0.083 ppm respectively). Elevated concentrations of Fe, Mn and As (20.3, 1.09 and 0.048 ppm respectively) were observed to decrease rapidly after the cessation of waste-rock dumping in a lignite pit-lake, and probably reflects a high rate of sedimentation, and the ability of the trace elements to adsorb onto this sediment. Lake stratification events did not play a significant role in trace element distribution at either site. This was attributed to a lack of lake biological productivity resulting in weak oxygen stratification at the sub-bituminous pit-lake, and the shallowness (3 - 10 m deep) and activeness of the lignite pit-lakes resulting in short-lived stratification events. After periods of settled weather, turbidity increased with depth at the sub-bituminous pit- r lake (10-21 NTU from 5-30 m). In the stratified sub-bituminous pit-lake, heavy rainfall resulted in higher epilimnion turbidity compared to the top of the hypolimnion (20 NTU vs. 13 NTU) reflecting surface water runoff (up to 1993 NTUs) and epilimnion mixing. The turbid substance from surface water runoff entering the sub-bituminous lake was dominated by the clay mineral kaolinite. Due to the flat, plate like structure and slow settling rate of kaolinite, its presence at the sub-bituminous site has resulted in long-lived lake turbidity. In contrast, turbidity at the lignite sites was much more variable ( 4-87 NTU), but decreased rapidly after periods of settled weather. These 'short-lived' turbidity events in the lignite pit-lakes, are due to a greater proportion of large, coarser particles entrained in pit-lake bound surface water runoff, than at the sub-bituminous pit-lake.

vi, 150 leaves : ill (some col.), maps (some col.) ; 30 cm.

2006Mulliner